Transformation and Microstructure Control in Boehmite‐Derived Alumina by Ferric Oxide Seeding

Single‐crystal seed particles of α‐Fe₂O₃ were produced by aging ferric nitrate‐derived precipitates. Addition of these particles to boehmite (γ‐AlOOH) sols at concentrations up to 10¹⁴ particles/cm³ of dried gel (4.2 wt%) reduced the θ‐ to α‐Al₂O₃ transformation temperature from 1215° to 1085°C as measured by DTA at a heating rate of 10°/min. The addition of Fe(NO₃)₃ solution to boehmite at an equivalent Fe₂O₃ concentration of up to 8.1 wt% lowered the transformation temperature to <970°C. The increased nucleation frequency afforded by Fe₂O₃ particle and salt additions permitted the development of fully dense submicrometer α‐Al₂O₃ at <1250°C. Transformation control in alumina is discussed in terms of nucleation frequency and epitaxial effects. 1988 The American Ceramic Society